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A Dipole for your CB Base Station?

Discussion in 'CB Antennas' started by Robb, Jan 6, 2009.

  1. Robb

    Robb
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    While looking for an antenna for my new Ham rig, I was looking at dipoles. I was looking on the Radiowavz website for a Windom-design 10-40 meter dipole antenna, and noticed that they had a page that had dipoles for CB/11 meters too.
    C.B. / 11 Meter

    I've never known anyone to have or use a dipole on CB in all of the time that I was on CB years ago. I'm not sure that any of you have used one - or still do. The boast of the dipole for CB is the same as it is for Hams; low noise due to horizontal polarity, high power capacity, stealthy, and easy to tune and upkeep. There are rules that one must use for installation; but not beyond the scope of anyone. SO; does anyone use a dipole - or have you used one in the past? Tell me all about it, if you would.
    Thanks!


     

  2. 74IN

    74IN
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    Dipoles do not have to be horizontal.

    Due to many CB users having mobiles, makes end fed base antennas common.

    The first dipole antenna I made was a copper pipe one. It was vertical.
     
  3. Moleculo

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    Sure, a lot of the folks here have used dipoles on 11m...why not? The guys that like to "talk skip" usually configure them horizontal or inverted V. The guys that use them for local configure them vertically. There used to be a few pictures of various peoples dipoles for 11m on this forum, but they might be gone now. They work just fine...of course they're not going to have the gain of a 5/8 wave vertical or a yagi, but they work the same on 11m as they do anywhere else.
     
  4. Robb

    Robb
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    Do the 11 meter.CB dipoles require a balun? Or is it a question of the SWR reading? I want to put up a CB dipole and see how well it works. I have a 1/2 wave vertical up already; so I want to make my own dipole with an SO-239 connector and some wire. How would you do it?
    Moleculo?
    Doc?
    Anybody?
     
  5. Moleculo

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    The answer to your question is "it depends". If you already have a 1/2 wave vertical and want to do a vertical dipole, you're likely to see basically the same performance. If your goal is to try and "talk skip" and install the dipole horizontally, then you are probably going to see some differences.

    I take it that when you said you want to "build it with an SO-239 and some wire" you mean that you're going to use coax for the feedline and want ot be able to just screw it into the dipole. In that case, all you have to do is solder one of the dipole wires to the center conductor, and the other to the outside barrel part of the connector. Of course, you have to figure out how to mount that stuff so it will last. You should use a 1:1 current balun, or make a choke balun by coiling up quite a few turns of coax (6-8 should do it?) and taping it together. Let us know if you need info on that.

    To be honest, it's almost easier to just buy a kit that has that part made for you. Something like this is relatively inexpensive and includes the balun and connections for the wire pre-built:

    http://www.k1cra.com/images/products/original/716.jpg

    You can get it at K1CRA Radio Store - W2AU 1:1 Balun - For Dipoles Details

    BTW, the balun isn't absolutely necessary, but it's a good idea to help prevent common mode current on the feedline.
     
  6. Wire Weasel

    Wire Weasel
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    Seeing as you already have a vertical, try the dipole as horizontal. On DX, signal polarity tends to twist around. A signal may be launched horizontally yet when it comes back down on the other end, it may be more vertical and you'll see a bigger signal on your vertical. Or it may come back down stronger in the horizontal plane. And if you talk with the station for many minutes, it can switch around back and forth while you're talking. So it is a good thing to have both capabilities and you can switch back forth between the two (with a two-position coax switch) and simply see which antenna is hearing better at any given moment.

    Dipoles will not require a balun unless you're experiencing RF feedback or spurious radiation from your feedline with a particular installation. In this event, add a 1:1 balun at the feedpoint. This changes nothing impedance wise, just isolates the coax so it cannot radiate any signal.

    good luck

    [​IMG]
     
    #6 Wire Weasel, Jan 6, 2009
    Last edited: Jan 8, 2009
  7. Robb

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    OK. Thanks guys - for the input.
    What are the advantages of an air core balun as opposed to a ferrite-core balun - besides the price? Let's say that we are going to bump up to a 100wicker heater for the CB at various times. Most of the cheaper baluns are air core - correct? The more expensive ones ($60-80) are ferrite cores. Do the air cores have a low limit with power?

    Is there a noticeable difference between the price and the on-the-air quality of air vs ferrite? No one seems to talk about this very much that I've noticed. And Wire Weasel; have you operated a horizontal CB dipole? Overall - did you like the quietness of the design? How does it compare in s-units to the 1/2 wave vertical ground plane at a given distance? Was it good for local QSO's as well as skip? Advantages/disadvantages/tradeoffs?
    Thanks!
     
    #7 Robb, Jan 6, 2009
    Last edited: Jan 6, 2009
  8. KB1TPX

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    My 2 cents....

    For what it's worth, I've used a horizontal wire dipole, in a "V" config. with great results and I now use a home brew vertical "galvanized conduit" dipole that will talk with the best of them. Back in the 60's, a buddy of mine made one 8'8" in either direction, it suited him fine til he got a "Super Magnum".
    There's always the eBay way: http://shop.ebay.com/?_from=R40&_trksid=m38.l1311&_nkw=g5rv&_sacat=See-All-Categories
    Or,
    You can make your own and just use the many calculators from various websites to determine the length according to the frequency that you need it for. Mine is 8'8" x2, which is good for the whole cb band and works well for SSB as well.
    10 Meter Dipole - N2UHC's Radio Stuff I used this basic design and constructed it to the length I needed.
    I will use it til I finally can fit a beam into my budget.
    Here's the finish product
     

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    #8 KB1TPX, Jan 6, 2009
    Last edited: Jun 18, 2009
  9. Wire Weasel

    Wire Weasel
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    Hi Rob,

    Yes I've used dipoles before. But dipoles (horizontal) are not inherently quiet antennas. They are only "more quiet" where the majority of signals present on a given band are vertically polarized.

    In general, there are two electronic types of antennas. Voltage point and current point. Dipoles and all vertical type antennas are in the voltage point family and are considerably more "noisy" than current point types. Two common examples of current point antennas are loops and off-center fed dipoles such as the G5RV and Carolina Windoms. I use a long horizontal loop cut for 80 meters, and it works on all bands 80 and above up to 6 meters. It has natural resonant areas in 80,40,20,17,15 and 11 meters and reaches 10 and 6 with a good tuner. It will not operate on the 60 meter band in 5 mhz without a full range outboard antenna tuner. There is tons of info on the net regarding loops, G5RV and C. Windoms. Simple to build wire antennas.

    A horizontal dipole, will work just fine for local contacts but the other station you're talking with must also be using a horizontally polarized antenna to achieve any distance. Otherwise you're working against a 90 degree polarization mismatch which will cut your distance working ability down more than 50%.

    Straight-strung "flat-top" dipoles are also largely bi-directional. They get out equally front and back but will have a null area going off each end. So if you use one, you need to decide which two main directions are most important to you and string the thing up for those directions. If you most need to talk north and south, then the ends of the antenna need to be tied up east and west.

    I use a loop because, particularly in a square shape, and largely so for a rectangular and triangular shapes, they will have a mostly omnidirectional pattern. The other factors being an all-bands-in-one type and they are more quiet against man made noise being a current point type antenna. Loops work on all the main bands above the lowest one it is cut for, and up to somewhere near 50 mhz.

    Like 7732GS said, a dipole can also be put up with only one elevated tie off point, the middle, making it into an inverted V shape. This will tend to make the radiated pattern somewhat more omnidirectional but will also reduce the bandwidth. It will be important to make both the legs as close to an exact 45 degree angle as you can and both leg ends should be the same distance off of the ground, to help achieve the best desired results.

    Wire antennas are a staple of many hams. They are cheap and easy to build and put up, and are stealthy. Very hard to drive by your house and even seen one. People have trouble spotting my 270 foot long 80 meter loop, which is attached to the top of my house and runs around the backyard, even when standing in my yard and I'm pointing it out to them. Even new copper tarnishes very quickly and shiney new copper will turn a dull brown in just a few weeks outside.

    [​IMG]
     
    #9 Wire Weasel, Jan 7, 2009
    Last edited: Jan 8, 2009
  10. 190

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    Does it matter which direction a guy strings these di-poles up ? north to south, east to west.?? is the ladder line or the round coax better.?? i was looking at the link 7732granitstate posted and it looks like some use both ladder line into rg8x coax. that right.?? 300 ohm ladder line into 50 ohm coax.?? does it matter where in the line this here bahlin hooks in.? im di-pole and wire antenna challanged and was just wanting to learn. i got a empty antenna spot on my 756pro2 that needs something plugged into it.
     
  11. W5LZ

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    A dipole is a pretty versatile antenna. It can be hung vertically, horizontally, and almost anything in between those two. Bent in to various shapes to fit the area you have to put it into. If it's high enough not to 'clothes-line' yourself, it'll probably work. If you can put it on top of a huge pole/mast/tower/tree, it'll probably work even better. A dipole (most common meaning being a center fed 1/2 wave antenna) is probably the most 'universal' antenna in use. They are not difficult to make, but do require 'tuning' unless you are just ridiculously lucky. That just means adjusting the length, basically.
    For HF dipoles, a balun just isn't all that necessary. Won't hurt, but in most cases also won't help either. Dipoles are also the 'standard' all other antennas are compared to. Several 'qualifications' to that, but still a 'standard' means of comparison.
    What you make a dipole out of depends a lot on what you happen to have on hand. The size of wire doesn't play any significant part other than to be strong enough to hold it's self up in the air with out stretching much. Anything smallet than about a 18 guage is sort of 'too small', concerns the mechanical 'strength' thingy, not electrical characteristics. Larger doesn't hurt, it just gets sort of unwieldy(?) at some point. Size of wire doesn't determine how much RF power it's capable of handling.
    The 'magic' number for a 1/2 wave antenna is '468'. Divide that by the freq in Mhz and you get feet. Adding a foot or two extra to make connections with is sort of a good idea. You'll have to tune it anyway, so any left over unused length gets done away with, no big deal.
    Dipoles are only difficult to make until you make the first one. Then, you have at least some idea of what's required, and the following ones are easier. Keeping both 'sides' of that dipole the same length is a very good idea. If you trim so much off of one side, trim the same amount off of the other side, or add to it the same amount.
    DO IT! Do it! (They're cheap too!)
    - 'Doc


    Quick-n-dirty!

    10 foot extension cord.
    Cut the receptical end off.
    Separate the two wires down to the plug end.
    Solder coax center conductor to one 'blade', the braid to the other 'blade'.
    Adjust length of each side the same amount till it works 'right'.
    Quit.
    That's about as 'dirty' as you can get. It'll work just fine.
     
    johnac likes this.
  12. Robb

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    Doc; where would the balun come in? That is a pretty neat down 'm dirty way you have there; but what would be the reason for adding the balun?

    OR - why is there a standing current left on the surface of the cable - if it has a means to load on the extension cord?
     
  13. W5LZ

    W5LZ
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    Rob,
    If you are going to use a balun, then the place in that extension cord thingy is between the coax feed line and the 'plug' connections at the center of the dipole. I just don't see any great need for a balun. The only purpose for a balun is to minimize any reactions from going from an unbalanced feed line to a balanced antenna. Since about the only thing affected by this balanced/unbalanced condition at HF and with an only 'semi-directional' antenna is the symmetry of the radiation pattern, and since everything else around that antenna is going to affect the symmetry of the radiation pattern anyway, what's the point? A balun can reduce any common mode currents on the feed line, but so can a simple choke. I typically have more unused feed line to wind that choke than I have spare baluns laying around. Just seems easier/cheaper and one less 'point of failure' to worry/wonder about when it isn't absolutely necessary to begin with.
    Baluns do have their uses. In the case of most HF antennas that 'use' just isn't there, or just not something to worry about. You don't have to balance it, just 'choke' it.
    - 'Doc


    (all puns intended)
     
  14. Robb

    Robb
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    Baluns are choke coils in paralell with the coax at the junction of the unbalanced coax to the balanced line; so the load can be distributed across the length of the wire - right/wrong? These baluns are wound at a rate of 1:1, 2:1, 4:1, 5:1, and upwards. Is the SWR of the line responsible for the choice of the balun? How would one figure out which is best - so that one wouldn't have to buy or build a gaggle of these things to make the antenna work? Would you use the SWR ratio as the guide to pick the right one? How is this accomplished?
    Thanks Doc!
     
    #14 Robb, Jan 7, 2009
    Last edited: Jan 7, 2009
  15. W5LZ

    W5LZ
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    A balun is a trasnformer with several 'windings'. How those windings are connected can affect the balanced/unbalanced thingy. And just like any trnasformer, the ratio of the number of 'turns' between primary and secondary can affect the impedance, change it x2, x3, x4, etc. The idea of that impedance transformation is to match the impedance of the feed line to the input impedance of the antenna. That's just like a one-way street. If you get it backwards, you got a really bad mismatch and high SWR. There really isn't any need for other than a 1:1 impedance transformer with the typical 1/2 wave dipole, since the 'classic' input impedance of a flat top dipole is something around 72 ohms give or take a little. A 2:1 balun (transformer) would make the feed line see something on the order of (2 x 72 = 144 ohms), or the other way around, about 36 ohms. About a 3:1 SWR in the first case, closer to a 2:1 in the second. A 1:1 balun would give something like 1.5:1 SWR. that 1:1 balun just gets closer to what's wanted. If you use that dipole for multiple bands, the impedance changes get really wild, and not really predictable. So unless there's a particular need for that impedance transformation ability, don't bother. One of those particular 'needs' is with a multi-element antenna such as a beam. When you add elements in fron/behind the driven element they lower the typical input impedance. The more elements, the lower it goes. Usually, you end up with an input impedance close to about 25 ohms. Then a 1:2 (backwards 2:1) balun certainly can be useful. (Remember that 'one-way' street thingy, don't get it backwards.) About the only time you even need to think about that is with a 'directly' fed beam. If that beam uses a 'match' of some kind, gamma, hair-pin, whatever, ~it~ takes care of that impedance matching/transformation. So a 1:1 balun would/could be useful.
    Using a beam is also one of those times when it's nice to be able to predict the shape of the radiation patter, where the signal is pointed, right? So it also means that using a balun to keep that balanced/unbalanced 'slewing' of the radiation pattern to a minimum is a good idea too.
    On the lower HF bands, under 20 meters, sort of, a dipole is seldom ever at a height that is going to allow any directionality. It's typically a sort of weird shaped, omnidirectional, 'lumpy', radiation pattern. 'Smoothing' those 'lumps' out isn't going to change the radiation pattern much, so why bother? That's why I say baluns aren't of much use on the HF bands. See?
    - 'Doc

    After seeing this post... posted, and all the spelling etc, mistakes I started to correct it. But there's so many, @#$$ with it, just read around the mistakes.
     
    338_MtRushmore likes this.

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